We seek to describe the host-relationships of the vector tick, Ixodes dammini, and to determine how the abundance and variety of vertebrate hosts may affect the intensity of transmission of the agents of Lyme disease and rodent babesiosis. We shall determine: (1) how the abundance of particular reservoir hosts of the tick-borne spirochete, Borrelia burgdorferi and the piroplasm, Babesia microti, relates to the intensity of transmission; (2) whether the abundance of certain verebrate hosts may largely regulate the abundance of the vector tick, Ixodes dammini; and (3) whether the presence of other vertebrate hosts may limit the geographical distribution of this tick. In addition, we shall (4) identify non-host factors that may affect geographical diversity in the transmission of these infections, including parasitization by a chalcid wasp, interaction with other ticks and the presence of genetic diversiy within I. dammini. We shall monitor the course of human infection and disease in sites where these zoonoses affect human health. We hope to build on these observations in order to identify the main factors influencing the force of transmission of these agents of human disease and, ultimately, devise means for protecting human health. This work may generate concepts applicable to an understanding of the transmission dynamics of vector-borne infections in general.